U.S. Pat. No. 4,990,168 to Sauer et al describes a two or more stage membrane and distillation process to separate and concentrate carbon dioxide from a destination tail gas mixture containing it in a high concentration of about 75% CO.sub.2. The nature and composition of the membrane material is described in detail, and polyimide, polyarimid, polyester, polyamide and cellulose acetate are noted as examples. The use of different membrane materials with different properties for application in different stages is not described.
Furthermore, although a process such as that described in the Sauer '168 patent may be useful in limited localities where gas mixtures containing carbon dioxide in a high concentration are available, such as the tail gas of a typical CO.sub.2 liquefier, such a process is of little value in the more numerous localities where gas containing carbon dioxide in a high concentration is not readily available, such as a fossil fuel combustion exhaust or limekiln vent gas. Therefore, the Sauer '168 patent does not address adequately the problem of providing high purity carbon dioxide in localities in which a source containing carbon dioxide in a relatively high concentration is not readily available.
U.S. Pat. No. 5,102,432 to Prasad describes a two or more stage membrane process to separate and concentrate high purity nitrogen from ambient air. This process relates to concentrating the less permeable component of a gas mixture to high purity by refining it by passing it through successive stages as a high pressure non-permeate phase. The Prasad '432 patent describes that a permeate stream of a downstream membrane is recycled to the initial feed and/or to the non-permeate phase of an upstream membrane. Also, the Prasad '432 patent specifies a range of membrane selectivities.
A need has continued to exist for improved processes and apparatus for providing high purity gases in an economical and efficient manner; in particular, an improved process and apparatus for providing local production of high purity carbon dioxide starting from abundantly available low concentration sources as mentioned above has been strongly desired.